Velcro strips are a cost reduction method of attaching seat coverings to polyurethane pads principally used by the automotive companies. These strips are attached to the polyurethane foam during the molding step simultaneously causing the cost reduction and a problem. The problem arises from the molded-in-place Velcro strips releasing air into the rising polyurethane foam during the molding step. This release of air produces large voids at the Velcro strip/foam interface adversely affecting the foam's quality, load bearing and adhesion to the Velcro strips. As an aid to understanding the problem, a brief description of the construction of a Velcro strip/polyurethane foam article composite follows:
The Velcro strip consists essentially of a network of molded plastic teeth and usually a nonwoven or knitted backing material. Only the backing material is important to the problem. For cost reasons, polyolefin fibers are predominantly used to make the backing fabric. Polyolefins, by their chemical nature, are non-polar materials characterized by low free energy surfaces. The nonwoven construction technique comprises piling fibers in a random manner to form a web. The fibers where they intersect are bonded together with a polymeric binder resulting in a vast number of air pockets, or voids, in the fabric. Knitted fabrics are constructed by looping the yarn together to form the fabric. The knitted fabric like the nonwoven fabric is characterized by a large number of voids. The problem would also be present with woven backing material.
In a process that takes place in approximately 40-50 seconds, the Velcro strips at ambient temperature are placed plastic mating teeth side down in the mold cavity. The mating teeth are covered by a thin plastic film to prevent coverage by the developing foam. The surface temperature of the mold is about 135.degree.-155.degree. F. depending upon the manufacturer's preference. Within 20 seconds after the strip is placed in the mold cavity, mixed liquid foam chemicals are poured into the mold. The pour pattern may or may not cover the Velcro strip with the liquid. The mold is closed and, within 5-6 seconds after pouring, the chemical reaction begins to convert the liquid into solid polyurethane foam. The rapidly expanding mass will fill the entire mold within 15-20 seconds after it is closed. During the filling process, the foam's viscosity and integrity is dramatically changing. It's viscosity changes from a liquid of a few hundred centipoise to a solid with a tensile strength of 10-15 lbs/in.sup.2 within 2-6 minutes depending upon the mold temperature and foam chemicals.
As a chemical entity, polyurethane is highly polar being characterized by a high free energy surface. This disparity in surface free energy with that of the nonwoven backing material prevents the wetting of the polyolefin fibers by the polyurethane foam, i.e. prevents the displacement of the air entrapped in the voids of the nonwoven material. To compound the problem, the entrapped air is absorbing heat energy from the mold surface and the exothermic reaction that forms the foam. This heat energy adsorption causes the air pressure in the voids to rapidly rise and blow out at the time the foam strength is beginning to develop. The result is large voids at the Velcro strip/foam interface. To describe the problem semi-quantitatively, instead of having 6400-10,000 attachment points/in.sup.2, the Velcro strip may have less than 100 attachment points/in.sup.2, based on good quality foam having 80-100 cells/inch while the voids are 1/8 to 3/4 inch in diameter.
One of the first approaches to solve this problem was to pretreat the nonwoven backing with various components of the liquid polyurethane foam composition. The idea was to displace the air by the liquid. It did not work because the highly polar liquid components were not able to wet the fibers.
A variety of surfactants were used to pretreat the nonwoven backing. Both anionic and nonionic surfactants were tested with partial success at levels from 0.1 to 1 wt.%. Nonylphenoxy poly(ethoxy)ethanol with an ethylene oxide content of 60 wt% significantly reduced the void size when tested at 0.2%, but the voids were not eliminated.
Solvent (acetone, ethyl alcohol, hexane, toluene and methylene chloride) extraction of the nonwoven backing fabric was also tried without success.
Both faster and slower, blowing and gelling catalyst changes were tried without success. Mechanical changes in the molds and altered pour patterns also were attempted without success. Even a vent directly above the Velcro strip to permit fast air release could not eliminate void formation.